In many mechanical and hydraulic systems it is often desirable to control the system by means of electrical signals. Consequently, in systems which operate with either compressed air or pressurized oil, it is necessary to provide an electromagnetic transducer which converts the electrical signal into a controlled fluid pressure which then actuates the mechanical equipment. One such transducer is a conventional electromechanical servovalve in which the electrical signals are converted into a mechanical force which, in turn, moves a mechanical valve to control the flow of fluid through the valve. It is especially desirable to make the mechanical movement of the valve directly proportional to the magnitude of the electrical signal which is applied to the valve so that a precise proportional control can be obtained.
Numerous prior art devices have been devised to convert an electrical signal into a proportional mechanical movement for operating a mechanical valve. However, such prior art devices suffer from several problems. For example, many of these devices are relatively complicated and, thus, are fragile and expensive to manufacture. The electromagnetic transducer in other prior art devices is difficult to seal properly to exclude pressurized fluid which leaks from the valve mechanism due to manufacturing tolerances and wear.
One relatively simple prior art device disclosed in U.S. Pat. No. 4,040,445, utilizes an electrical "voice coil" drive to directly actuate a mechanical servovalve, thus simplifying the valve construction. The same device also utilizes metal bellows to isolate the force transducer from the valve to prevent contamination by leaking fluid. This device overcomes some of the aforementioned problems, but it has a limited lifetime and reliability problems. More particularly, it has been found that after a relatively short operating time, the device either jams or becomes electrically shorted and nonfunctional.
In addition, it has been found that the aforementioned prior art servovalve is difficult to adjust. More particularly, such an electromechanical servovalve must be adjusted so that the force transducer is electrically centered and the mechanical valve portion is mechanically centered. In the disclosed device, the mechanical valve mechanism is attached directly to the electromechanical force motor. Consequently, any adjustment must be a compromise between centering the servovalve and centering the electromechanical force motor. Such a compromise often causes inefficient operation and gives rise to nonlinearities in that the force transducer is not precisely centered when the mechanical valve is in its neutral position.
Accordingly, it is an object of the present invention to provide an electromechanical servovalve which is simple in construction.
It is another object of the present invention to provide an electromechanical servovalve which is reliable in operation and has a long life.
It is still another object of the present invention to provide an electromechanical servovalve in which the operation of the valve is proportional to the electrical signals applied to the force transducer.
It is still another object of the present invention to provide an electromechanical servovalve in which the operation of the valve is linear with respect to the electrical signals applied to the force transducer.
It is yet another object of the present invention to provide an electromechanical servovalve which is resistant to jamming or shorting.
It is a further object of the present invention to provide an electromechanical servovalve which utilizes a voice coil electromagnetic force motor.
It is still a further object of the present invention to provide an electromechanical servovalve in which the force transducer and the servovalve can be adjusted separately.